Oxidation processes are common in industrial processes for preparing various types of substances. Selectivity towards and purity of the desired product, inherent safety and minimization of waste & environmental emissions are constant challenges that industrial oxidation processes face. As such, improvements in oxidation processes and the reaction systems that conduct the oxidation are continually being sought.
The catalytic liquid-phase oxidation of para-xylene to terephthalic acid (hereafter, TPA) with Co/Mn/Br based catalyst, known as the Mid-Century Process, was developed in the 1950s. In 1965, a hydrogenation step to purify TPA was added to help remove 4-carboxybenzaldehyde from the reaction product through conversion to water-soluble para-methylbenzyl alcohol and crystallization.
Another related reaction scheme produces crude TPA at relatively mild oxidation conditions with a metals/bromide catalyst system. The production of isophthalic or TPA from the corresponding xylenes has also been proposed using acetaldehyde as a promoter for the reaction. Since this process does not use bromine as a catalyst promoter, less exotic reactor materials are suitable. Another known process is characterized by the simultaneous oxidation of para-xylene and methyl para-toluate at mild temperature and comparatively low pressure without acetic acid solvent.
However, all of these processes share a common shortcoming—an inadequate or non-optimal O2 mass transfer rate in the liquid phase. The mass transfer is accomplished in a stirred liquid phase reactor, wherein the air is vigorously bubbled through the liquid phase. The crude TPA solid produced via this process is separated and further purified in a subsequent stage to reduce the 4-carboxybenzaldehyde content. Further, roughly 5% of the acetic acid entering the liquid phase reactor is also oxidized (e.g., burned) in this process. Therefore, there still remains a need in the art to have an improved process that provides increased O2 mass transfer rate for the production of TPA and other oxidation reactions similar to the ones discussed above.